Transparent panels such as glass panels, especially panels of toughened glass, are widely employed in high-rise structures, and there is a need to inspect them to check for the presence faults. In particular, glass panels are subject to Nickel Sulphide (NiS) inclusions and other defects (faults). If such checks are not carried out, the Nickel Sulphide inclusions may cause the toughened glass to fracture, and the glass may shatter with potentially disastrous consequences, both for people inside the high-rise building, and on the ground beneath. Ideally, checks for inclusions and other faults should be performed during the manufacturing process for quality control (QC), and also in-situ once the panels are installed in the structures for quality assurance.
Several techniques are known for testing glass panels to observe inclusions.
A first of these is visual inspection, carried out using a microscope (e.g. a portable microscope), scanning the whole glass panel point-by-point manually, or automatically using a CCD (charge coupled devices) camera and image processing software based on a threshold. Unfortunately, this process is very slow, due to the limited field of view (FOV) and depth of focus (DOF) of the observation process. NiS is black/grey in colour and only reflects light weakly. NiS inclusions are therefore difficult to observe against the generally high intensity of background light caused by reflections and scattering of light due to surface contamination and scratches. Furthermore, many glass panels are printed with black dots to reduce the intensity of sunlight transmitted through them, and it is very difficult to observe the tiny dark inclusions in the shadow regions caused by the dots.
Secondly, there are photographic techniques in which a portion of the glass is photographed in controlled lighting conditions, and the image is magnified and visually examined to detect inclusions. Though the image is taken on site, the magnification and inspection are performed somewhere else. Thus, such techniques are not suitable for in-situ testing and are inconvenient even as a part of a QC mechanism.
Thirdly, there are techniques in which coherent laser light is directed at the front surface of a glass panel, and measurement are made of light scattered back towards the front surface by the inclusions, and also of light scattered forward by the inclusions and then reflected by the rear surface of the panel towards the front of the panel. Comparing these two signals makes it possible to determine the location, size and depth of the inclusion by signal analysis. However, in this technique only the area of the panel illuminated by the laser beam is measured. Furthermore, most of the energy of the laser is not utilized, and either completely penetrates the glass panel, or is reflected by the glass surfaces. Furthermore, contamination on either surface of the glass can cause light scattering, which induces noise in the measurement, and hence in the inclusion image obtained.
Fourthly, there is known a technique employing Raman spectroscopy, in which a laser is used as a light source and a nitrogen-controlled charge coupled device is used as the detector to obtain a Raman spectrum. By comparing the Raman spectrum of a measured glass panel with those of specific substances, the presence of inclusions such as NiS can be identified. However, present technology only allows this technique to be used in a laboratory because it requires critical control of implementation conditions. For this reason, it is better suited to measuring the composition of previously discovered inclusions, rather than for an initial inspection of a glass panel which may or may not include inclusions.